When an event occurs, an image monitoring system performs a pre-event recording function for recording an image at the previous time of occurrence of the event. In order to perform the pre-event recording function, a pre-recording memory is provided for each channel to record an input image signal. In this case, when an image signal having a large data amount is input, there is a limit to the memory size. Therefore, an image signal of a certain period at the most previous time is deleted and a new image signal is recorded to ensure recording for a short time. Like this, in the related art, since the pre-recording memory provided for each channel has a limited capacity, when an image signal having a high data rate is input, the image recording time is reduced. Also, it is inefficient because a pre-recording memory is provided even for a channel that is not currently used.